1. Field of the Invention
This invention relates generally to the field of microphones, and particularly to a method and apparatus for sensing audio signals that are sensitive to the audio signal but are insensitive to background noise.
2. Background Art
Prior art microphones generally operate under the principle of transducer action. Acoustic waves generated by one""s vocal chords impinge on a transducer element, such as a diaphragm, piezo-electric crystal, or capacitor plate. The displacement of the transducer element caused by the acoustic waves is transformed into a corresponding electrical signal which is amplified for presentation or recording. For example, piezo-electrical crystal microphones, variable-resistor class microphones, dynamic microphones, and condenser (also known as capacitor or electret) microphones all operate under the transducer theory that relies on pressure waves in the air or contact vibration sensing to convert speech into electrical signals.
One major drawback of prior art microphones is that they are sensitive to background noise. Accordingly, these prior art microphones are often less sensitive to the intended audio source. Furthermore, these prior art microphones are not easily adapted for use in many different situations because of their fixed placement. For example, many prior art microphones are mechanically mounted in a fixed location so they will not be able to receive the acoustic waves from a moving audio source in a consistent manner. As a further example, other prior art microphones are portable in that they may be worn by the talker, but these microphones suffer from the inconveniences associated with entangled and cluttered wires. Further, wireless microphones are subject to RF interference.
These prior art microphones typically exhibit tolerable performance in high signal and low noise environments. However, when the noise in an environment increases, the performance of these microphones degrades appreciably. In fact, in high noise environments, the signal to be recorded is often muffled or imperceptible because of the surrounding background noise. The signal to noise ratio (S/N) can degrade to a point that no meaningful audio information is recovered after demodulation.
Furthermore, as one increases the sensitivity of these prior art microphones to improve signal quality, these microphones also become more sensitive to the background noise. In other words, prior art microphones do not have the ability to differentiate between the audio signal and background noise.
Accordingly, there remains a need for a method and apparatus for sensing an audio signal that is sensitive to the audio signal but insensitive to background noise, and which provides improved signal quality.
According to one aspect of the invention, the present invention provides a method and apparatus for sensing audio frequency pressure modulation of the moisture content of the atmosphere caused by a moisture-laden audio source, such as a live human voice. As a result, the present invention is sensitive to the humid source of the audio signal to be detected, but insensitive to surrounding background noise. The present invention virtually eliminates sources of background noise by detecting only the amplitude modulation of a light signal caused by water vapor or moisture that is modulated by the audio signal to be detected. The audio signal modulates the water vapor or moisture in a human being""s breath.
A light emitter, configured to operate in a water-vapor absorption band, sends a light beam through a region containing this moisture and a corresponding light detector detects the received time-varying amplitude of a light signal that has escaped absorption by the water vapor or moisture. The light intensity or amplitude is time modulated by the variation in the moisture content caused by the voice of a person speaking (hereinafter referred to as a talker). Since background noise does not noticeably affect the moisture that is local to a talker""s mouth, the present invention does not detect the background noise, thereby increasing signal quality of the received audio signal.
According to another aspect of the invention, unlike the prior art that teaches operating a laser at wavelengths outside of the water-vapor absorption ranges to avoid absorption of the light beam, the present invention configures its light emitters, such as lasers, to operate specifically within a water-vapor absorption range. Moreover, the present invention employs water vapor or moisture that is modulated by the talker""s voice to modulate the light beam. In a conventional system that has a laser operating outside a water-vapor absorption band, a talker""s voice does not affect the light beam. In other words, a talker""s voice does not modulate the light beam to an extent that is detectable and from which audio information can be extracted.
According to another aspect of the invention, a light beam is directed near and in front of the talker""s mouth so that the present invention is insensitive to background noise that degrades the performance of prior art microphones. Moreover, because the present invention is configured to detect acoustic waves that are near and extremely local to the talker""s mouth, noise from other sources are not detected by the present invention.
Environmental conditions surrounding the talker, such as humidity or air conditioning, that do not affect prior art microphones can affect the signal quality of the audio signal detected by the present invention. Consequently, according to another aspect of the present invention, a second light emitter, second detector, and a differential amplifier are employed to measure the contribution of environmental conditions to the audio signal and to compensate the audio signal by subtracting the environmental contribution from the audio signal.